Okay, this is not completely a NFA question, but here goes. I want to do a SBR and found the upper I like. Can I buy it and use it on an AR lower that I have changed out the buffer tube to a pistol buffer until the stamp arrives. Also, can I then put it one a rifle lower or switch out the buffer back to the stock buffer once I have the stamp? I thought that for the lower to be considered a "pistol" it had to be registered as such, just changing the buffer tube doesn't do it.
Thanks for the help.